The present disclosure is directed to circuit components in optical (or fiber optical) systems. More particularly, the present disclosure is directed to integrated photonic transmitters, receivers, and transceivers providing a wavelength division multiplexer or de-multiplexer with a volume holograph waveguide adapted for use in optical, or photonic, circuits.
Fiber optical systems generally refer to the medium and technology associated with the transmission of signals in the form of light pulses, or photons, along a glass or plastic fiber. Optical systems are distinguishable from electrical systems using conventional electrically conductive wires, such as copper wire, in the transmission of electrical signals. Optical systems also include advantageous capabilities over electrical systems. For example, electrical signals interact with each other and their environment. This results in a need for non-intersecting and spaced-apart electrical wire links between electronic devices or electrical components. In contrast, photons generally do not interact with each other, and this leads to the possibility of different photonic signals sharing the same optical fiber. Accordingly, optical systems are adapted to carry much more information than electrical wire and are subject to far less electromagnetic interference.
Optical systems typically combine different photonic signals onto the same optical fiber, or separate photonic signals carried on the same optical fiber, with a generally similar basic circuit structure. For example, an optical signal can be generated with an optical transmitter. Several optical transmitters can be used to each generate a particular optical signal. Separate optical fibers connect each of the transmitters to an optical multiplexer. The optical signals travel along the optical fiber to inputs of the optical multiplexer. The output of the optical multiplexer is coupled to a single optical fiber. The optical multiplexer is a photonic circuit component that combines several photonic signals into a single photonic transmission that can be carried on the single optical fiber. In order to separate the single photonic transmission on the single optical fiber, the optical fiber is coupled to the input of an optical de-multiplexer. The optical de-multiplexer is a photonic circuit component that separates a single photonic transmission into the individual photonic signals. The outputs of the optical de-multiplexer are coupled to optical fibers that lead to optical receivers. Each of the individual photonic signals is carried on its respective optical fiber to the corresponding optical receiver.
In general, the multiplexer or de-multiplexer in the basic optical structure performs a type of wavelength division multiplexing, or WDM. Wavelength division multiplexing is a fiber optic technique that employs light wavelengths to transmit photonic signals in parallel on the same optical fiber. Wavelength division multiplexing has enabled optical service providers to meet consumer demands for ever-increasing bandwidth. Wavelength division multiplexing uses several to many channels (also known as lambdas or colors) to provide high capacity bandwidth across the optical system or optical network. Each channel carries an individual photonic signal providing the same bandwidth per channel in a single photonic stream. The channels are de-multiplexed at the end location.
Wavelength division multiplexing is used to carry many different types of data on the same optical fiber. One common type of wavelength division multiplexing is dense wavelength division multiplexing, or DWDM, and can use around 160 different channels to provide high capacity bandwidth. Another type of wavelength division multiplexing is coarse wavelength division multiplexing, or CWDM, which can scale far fewer channels than DWDM because the channels are spaced further apart than in DWDM. However, coarse wavelength division multiplexers have advantages in that they are less temperature dependent, do not require more expensive components, and are generally less expensive than DWDM.
Several devices can be used to provide the multiplexing or de-multiplexing functions in the above-described circuit structures. For example, circuits of the related art can use thin film filters (TFFs) or arrayed-waveguide gratings (AWGs). TFFs offer powerful and potentially channel-specific passband control, but typically lead to assembled systems of daisy-chained discrete filters. As the number of channels increase, circuits using TFFs dramatically increase in both the size of the component package and the fabrication complexity of the circuits. AWGs are well suited to provide a large number of channels, but have a limited ability to tailor passband and efforts to flatten the passband have resulted in insertion loss of several decibels. AWGs are also relatively delicate devices that can be rendered inoperable if mishandled or subject to extreme conditions, and are relatively difficult to manufacture because of their low yield rate. Another type of device, the holographic Bragg reflector (HBR), includes many advantages over TFFs and AW/VGs in multiplexing. HBRs are durable, less temperature dependent, relatively small, and exhibit flattop channels. HBRs include many of the better features of TTFs and AWGs while avoiding many of their limitations. Recently, it has been proposed that HBRs can provide suitable multiplexing and de-multiplexing in the above-described circuit structures.
Regardless of the components used, the above-described circuit structures suffer from disadvantages. For example, all include mechanical connections between the circuit components and include mechanical couplings of optical fiber. The connections can cause losses in the signal between components and are prone to mechanical failure. Additionally the spaced-apart components connected by fiber create relatively large structures. The footprint of assembled circuit structures is limited by the minimum bending diameter (approximately 5-centimeter) required for long-term mechanical reliability of standard telecommunications fiber. Many of the multiplexer devices are not suited for a wide range of temperatures, and the optical fiber connections tend to degrade in extreme conditions for all of the above structures. Also, temperature changes cause wavelength drifts that prevent the use of multiplexing devices in extreme conditions. Optical circuits used in such extreme conditions are often limited to one photonic signal per fiber, which increases costs, complexity, maintenance, and space requirements.
Accordingly, there is a continuing need to provide space-saving circuit structures and circuit components that provide excellent performance in a variety of conditions that are also relatively inexpensive to manufacture and maintain.